Salam
I want put out there, not addressing anyone, but in general, Quranic Arabic transcends today's Arabic grammar. Grammar later was developed after Quran, and some of is consistent with Quran and some of it isn't.
Wali in 5:55 is singular, though, it describes more the one entity, but that's purposeful, to emphasize the unity of the Welayat there.
Quran says "and we did not find more then one house from the submitters" but else where Quran says "a people of a house that will take care of him and wish him well"- so which expression is right? The reality is they both are and it's about which sounds better in the instance.
When it comes to Al-Qurba 42:23 has an odd form of that. What do I mean? Everywhere else Qurba is used in Quran, it's preceded either by "Dhul" or "Ulul", possessor and possessors respectively.
Ikhwanal Muslimeen is known in the west as "Muslim brotherhood", even though the translation directly would be "Muslim brothers".
In Lord of the Rings, if you recall "fellowship of the ring" refers to a group of people. "fellows of the ring" just doesn't ring well.
In two other verses, Ulul-Qurba or Dul-Qurba makes sense because it's aligned with "the Messenger" and so you know it refers to the Messenger and those who hold kinship/close relative relationship to him.
The question is why didn't it say "ulul-Qurba" here or "dhul-Qurba"? In 42:23 I mean.
It's because Mohammad (s) is included. But they are the closeness to each other. Same way "fellowship of the ring" has a better ring to it, "Al-Qurba" means there is they are very close to each other family wise, but because this closeness transcends blood relationship at the same time the reward he is accused of has to do with that, it's a very nice expression and includes Mohammad (s) in it. It's mean they are the nearness to one another - the Ahlulbayt (a) are bound to each other by each other.
People can take my word for it or others, or do research if interested.
42:23 to me is undoubtable clear proof of Mohammad's (s) and his family's (a) unity and that they share his station in terms of remembrance and how much he is to be loved.